1. Field of the Invention
The present invention relates to an exposure method, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
An exposure apparatus is employed to manufacture devices such as a semiconductor device. The exposure apparatus illuminates an original (also called a mask or a reticle) by an illumination optical system, and projects the pattern of the original onto a substrate (for example, a wafer or a glass plate) coated with a photosensitive material by a projection optical system, thereby exposing the substrate. With this operation, a latent image is formed on the photosensitive material and visualizes into a physical pattern by a development process. Oblique-incidence illumination methods for obliquely illuminating the original are known to miniaturize a pattern formed on a substrate. Annular illumination and quadrupole illumination, for example, are known as the oblique-incidence illumination methods. In the annular illumination, an annular light intensity distribution is formed on the pupil plane of the illumination optical system. In the quadrupole illumination, a light intensity distribution with four poles is formed on the pupil plane of the illumination optical system. A light intensity distribution formed on the pupil plane of the illumination optical system is often called an effective light source distribution. Note that the pupil plane of the projection optical system is conjugate to that of the illumination optical system.
In the following description, a direction is defined with reference to an X-Y-Z coordinate system that has its X-Y plane parallel to the substrate plane and has its Z-axis in a direction parallel to the optical axis of the projection optical system. If the distributions, in the X and Y directions, of the effective light source distribution have a difference between them, a difference (line width difference) may occur in line width between a pattern formed to extend in the X direction on a substrate and a pattern formed to extend in the Y direction on the substrate. Japanese Patent Laid-Open Nos. 2004-247527 and 2006-19702 disclose methods of adjusting the effective light source distribution.
Even if the effective light source distribution is ideally adjusted, a line width difference may occur due to factors other than those associated with the exposure apparatus. It is difficult to reduce such a line width difference by eliminating those factors.